The word 'pagan' started as the Latin 'paganus', a term used by the Romans to describe a civilian or country-dweller. By the 14th century Christians were using paganus to describe someone who was not a soldier of god and the modern word pagan is thought to have first appeared in the 18th century. It carried a negative meaning and some Abrahamic religions still refer to Christians as Pagans because they believe in the Trinity and not one god. Today it equates in many people's minds to the modern concept of 'The Old Religion'.
'Druid' is more vague. The first use of it I have found is by the Roman Tacitus who was writing in the first century AD. It isn't clear what he meant, it could have been priest but could equally well have been an elder or faith advisor. Today it almost always refers to a Pagan priest.
Let's go back to 100BC and try to identify what belief systems prevailed then. They were almost certainly centred on an attempt to understand nature and a desire to live in harmony with the world. Remember there was no clear understanding of the natural forces that ruled their lives and they attributed various aspects of these to supernatural entities. We have plenty of good evidence for this. The Romans were great ones for adopting local deities and they tell us the Romanised versions of two river gods in our area, Verbeia for the Wharfe and Belisama for the Ribble, we don't know what the Celtic names were. We can only imagine what their version would be if someone drowned, almost certainly they would assume that the deity associated with that particular place had decided to take a life. Springs, pools and even bogs were seen as being connected to another world. Archaeologists have found deep shafts with votive offerings in the bottom and sites like Flag Fen near Peterborough were centres where votive offerings were cast into the water. Funnily enough, all the rivers where such associations exist have been found flow Eastwards.
As for who guided these rituals the short answer is that we don't know. Forget everything you have ever heard about Druids. The modern Druidic and Bardic movements are an 18th century invention. The one thing we are certain of is that whatever the religious practices were, nothing was ever written about them and no evidence survives. The archaeology suggests that the chief of a social group was regarded as the spiritual leader, that he was elected and that if old age or some other shortcoming affected him he was ritually killed and another leader appointed. As for the sex of leaders, we know that a strong woman called Cartimandua was leader of our tribe, the Brigantes, from about 43 to 69AD and Boudica led the Iceni tribe, dying around 60AD.
In case you're wondering why I illustrate this piece with a picture of Castle View, I was told in all seriousness in the 1960s that a coven of witches existed there.
Castle View in 1978.